13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- 3.2.4. Reduction in fatigue threshold by absorbed hydrogen Figure 10 shows the result of the crack growth test. The details of the test method are found in ref. [19]. In the hydrogen-charged specimen, the reduction in the threshold stress intensity factor, ΔKth, is clearly shown. The reduction in ΔKth is one of the causes of the reduced fretting fatigue strength by a hydrogen charge. The fretting fatigue limit of both the hydrogen-charged and uncharged materials can be quantitatively evaluated by the model based on the ΔKth [19]. 5 6 7 8 910 20 30 10-12 10-11 10-10 10-9 10-8 10-7 Solution heat treated SUS304 In air Uncharge H charge In H2 Uncharge H charge ΔK(MPam(1/2)) da/dN (m/cycle) (a) Specimen and test method (b) Crack growth curves Figure 10. Effect of hydrogen on crack growth threshold of SUS304 4. Effect of work-hardening Work-hardening is a fundamental method of increasing the strength of austenitic stainless steels. Strengthened austenitic stainless steels are frequently used for high-pressure components. A 40% plastic strain was applied to the solution heat-treated materials by the tensile test at room temperature. The mechanical properties of the work-hardened austenitic stainless steels are shown in Table 2. Figure 11 shows the result of the fretting fatigue test of the work-hardened SUS304. When the fretting fatigue strength of the uncharged material in air was compared between the solution heat-treated material (Fig. 2) and work-hardened material, it was found that the work-hardening improved the fretting fatigue strength (○ in each graph). However, the reduced fretting fatigue strength due to hydrogen was almost equivalent between the solution heat-treated material and the work-hardened material. The effect of the work-hardening was suppressed by the hydrogen. This indicates that special consideration is required in the design of hydrogen equipment made of work-hardened austenitic stainless steels. 5. Effect of surface roughness For mechanical components requiring a gas tightness, surface roughness is one of the design factors. Figure 12 shows the fretting fatigue S-N curves of SUS316 in which the specimens have two Table 2. Mechanical properties and Vickers hardness Material Conditions Proof strength σ0.2 (MPa) UTS σB (MPa) Elongation δ (%) Vickers hardness HV Solution heat-treated 294 667 60 242 SUS304 40% pre-strained 955 1027 26 358 115 49 15 t = 6.9 2 A Detail of A 60° strain gage
RkJQdWJsaXNoZXIy MjM0NDE=